Plants vs. Zombies: Battle for Neighborville

Medals are collected in the PVE story mode areas, you can view the amount of medals you currently have by going to the region stats in the menu.

You can view the available medals once you are in a story level, simply press and on the map tab, press until you are on the medals screen. This lists the medals you can obtain in the area, and range from completing missions, collecting gnomes and chests, to destroying X amount of specific objects or climbing specific locations.

You should be able to get the amount of medals needed for the achievement simply by completing the main story quests in each area and then mopping up the easy destructibles and sightseeing achievements. There are plenty of guides on youtube if you get stuck with any of them.